Spectroscopy is commonly used as a tool for analyzing impurities in gases. Significant problems persist, however, when the impurity of interest interacts with the surface of the sampling system and of the cell which is generally used to contain the sample during analysis. An impurity which absorbs on or reacts with the surface may appear lower in concentration than it really is. Alternatively, if the system has previously been exposed to the same impurity so that the impurity is reversibly bound to sampling cell surfaces, then the system may contribute impurity to the gases being analyzed so that the same may appear higher in concentration than that actually contained in the sample under investigation.
The detection of trace amounts of water in ultra high purity gases is particularly difficult. Background signals appear through moisture release from surfaces of the system. Moisture also tends to react with other components, and to adhere to surfaces. System considerations include spurious effects caused by cell wall absorption-desorption, atmospheric interference.
Many commercial applications require specialty gas such as semiconductor nitrogen gas to have certified water levels below 10 ppb. Small of amounts of impurities in such process gases have been shown to drastically effect both the yield of final products, especially when used in severe conditions including high temperature operations.
Accurate measurement techniques have been somewhat enhanced by taking precautions considered standard in trace moisture analysis, such as the use of high quality electropolished stainless steel for constructing the gas handling system while avoiding materials known to contribute large amounts of water vapor, such as certain polymers. In addition, the sample cell can be heated to remove absorbed water and by maintaining the cell at elevated temperatures, the speed of absorption/desorption processes can be increased and equilibrium reached more rapidly and with less sample gas being consumed. This procedure is somewhat inconvenient, however, and has never been demonstrated to be sufficient to reach trace moisture levels in the realm of 10 ppb.
As noted in an article entitled Multipass Absorption Cell Design For High Temperature UHV Operation, R. D. Shaffer, et al., Applied Optics Vol. 28, No. 9, pp. 1710-13 (May 1, 1989), tunable diode lasers have been employed in a heated multipass absorption cells to give a noise equivalent concentration below 10 ppb. However, in this work, calibration was effected using ppm level moisture standards and a reduced pressure (10.sup.-3 torr) in the cell, whereas actual measurements are made at 10 torr in the cell. No data are presented on background moisture levels at this pressure. Obtaining adequately low and reproducible moisture levels under measurement conditions may be considered the principal difficulty in most problems of moisture measurement. The noise equivalent concentration quoted by Schaeffer et al. is characteristic primarily of the performance of their optical system (and, indeed, indicates a very good performance) and does not characterize fully the performance of the cell itself. No effort was made by Schaeffer et al. to optimize the flow pattern in their cell. In such a design, multimirrors are employed within the spectroscopic sample cell to effectively establish the sample cell volume within the system. Sample gases are passed within the cell while the analytical light beam is reflected multiple times to define the probe volume.
Spectroscopic techniques are frequently considered suitable for the analysis of reactive or corrosive gases because they rely on the interaction of a light beam, rather than any other physical probe, with the sample gas. This eliminates the possibility that an aggressive matrix gas will attack the sensor, producing erroneous readings or, at a minimum, reducing its lifetime. Spectroscopic techniques are also able to distinguish various impurities from one another on the basis of the wavelength of light at which interaction occurs with the impurity, so that no external separation means, (such as a chromatographic column) which might also be attacked by the matrix gas, is required.
Notwithstanding the above discussion, it is necessary, when using spectroscopic analytical techniques, to have a gas cell which can contain the sample gas and which is equipped with windows capable of transmitting the light being used. As noted above, for maximum analytical sensitivity, it is frequently also necessary that the cell be equipped with mirrors so that the light may pass through the sample cell more than once. In general, it is necessary to place these mirrors inside the cell where they are exposed to the matrix gas. If the sample gas reacts with the cell or its components, a reduction in performance can result should the windows or mirrors be obscured by deposits or the reaction products vapors into the gas phase. In particular, if reaction with the matrix gas leads to particulate deposits or to an increase in surface area it can be expected that the cell will exhibit an increased tendency to adsorb water vapor and subsequently release it, giving rise to considerable interference effects.
A solution to the above problems is generally approached by choosing cell and window materials which are relatively immune to the sample gas in question. For example, a corrosion resistant alloy such as Hastelloy is a good choice for a cell intended for HC1 analysis. However, such alloys are expensive and the windows, mirrors and/or sealing and mounting materials used to place them in the cell may still prove vulnerable to attack. In addition, this approach does not specifically address interference and background difficulties which may arise as a result of outgassing, desorption or other release of moisture or other volatile materials from the cell surfaces. Although, as previously noted, heated cells have been employed to reduce adsorption/desorption problems, this approach is not always sufficient and may accelerate destructive interactions between the sample gas and the cell.
It is generally recognized that a cell with inlet and outlet at opposite ends is purged of impurities more efficiently than one in which both connections are at the same end, but this simple consideration is not sufficient to effectively purge all the important regions of the sample cell.
It has been determined that accurate trace impurity measurement is exceedingly difficult in light of the interactions as noted above between the gas and walls of the spectrographic cell. Problems resulting from such interactions are particularly acute when stable vortices occur in the flow field. The sole mechanism for purging these vortices is diffusion, a characteristically slow process. As such, if no stable vortices were to occur in the flow field, the entire cell would be continuously purged by the same gas minimizing cell wall interaction and greatly enhancing the opportunity for accurate trace impurity measurement.
It is thus an object of the present invention to provide a spectroscopic sample cell design which greatly enhances the opportunity for accurate trace impurity measurement.